The present invention relates to a fail-safe system for an automotive internal combustion engine in which a throttle valve for controlling the flow rate of intake air of the engine is driven through an electro-mechanical actuator such as a motor, or more in particular to a fail-safe system which effectively functions when the throttle valve of an automotive gasoline engine has stuck.
Automotive engines are now required to satisfy triple major requirements. It must meet the emission gas control regulation and also the requirement of fuel economy, in addition to the dynamic performance. In an automotive engine, whose operating conditions vary over a wide range, fuel flow, ignition timing and etc. are controlled by a computer to satisfy these requirements. Further, an optimum engine control with high accuracy cannot be effected in satisfactory manner any longer by a conventional system in which the motion of an accelerator pedal directly corresponds to the motion of the throttle valve. In view of this, what is called a drive-by-wire system has been suggested, in which the motion of the accelerator pedal is detected by a sensor, the output of which and various parameters representing driving conditions are used by an actuator to drive the throttle valve.
A drive mechanism of the drive-by-wire system comprises a sensor for detecting the amount of depression of the accelerator pedal, a drive circuit for producing a valve drive signal corresponding to the output of the accelerator pedal sensor, and a step motor for opening the throttle valve in accordance with the drive signal. Another system may further includes a gear mechanism for transmitting the rotation of the step motor to the throttle valve.
The throttle valve, which directly controls the engine output, is required to have a very high safety. The drive mechanism of the drive-by-wire system, for its considerably complicated mechanism compared with the conventional system, must be equipped with more safety measures against faults. Especially in the case where the throttle valve sticks while the engine is in operation, the output control of the engine becomes impossible and therefore it is necessary to provide a fail-safe system against a runaway or engine stall.
The Japanese Patent No. JP-B-58-25853 filed for a patent by Toyota Motor Co., Ltd. with Japanese Patent Office on May 23, 1975 and published on May 30, 1983, discloses a fail-safe system comprising mechanical separation means such as an appropriate clutch between the actuator and the throttle valve, whereby the throttle valve is separated from the actuator by the clutch in the event that the throttle valve has stuck and throttle valve is returned to the full-open position by the force of spring.
This prior art system provides satisfactory fail-safe means to the extent that once the throttle valve has stuck, the engine becomes to be idling condition and thus the car is prevented from running away. If the engine is fixed to an idle state, however, the car cannot drive any longer. In the case where the car comes to a standstill on a road in the midst of a desert or out of town, it is difficult to seek a help or drive to a nearby service station on its own.
It is thus desirable that even when the throttle valve has stuck, the engine output can still be controlled so that it may manage to drive to a nearby house or service station for repair.